The present invention pertains to the field of plasma recombination lasers.
The idea of producing a population inversion during plasma recombination was first proposed in an article entitled, "Negative Absorption in a Nonequillibrium Hydrogen Plasma", Sov. Phys. JETP, Vol. 18, 1964, pp. 998-1000 by L. I. Gudzenko and L. A. Shelepin. Experimental observations of lasers of this type were made in 1977. The population inversion mechanism responsible for these lasers has been discussed in an article entitled "Recombination Lasers in Expanding CO.sub.2 Laser-Produced Plasmas of Argon, Krypton, and Xenon", Applied Phys. Letts., Vol. 31, 1977, pp. 334-337 by W. T. Silfvast, L. H. Szeto and O. R. Wood and in an article entitled "Recombination Lasers Utilizing Vapors of Chemical Elements. I. Principles of Achieving Stimulated Emission Under Recombination Conditions", Sov. J. Quantum Electron., Vol. 7, 1977, pp. 704-708 by V. V. Zhukov, E. L. Latush, V. S. Mikhalevskii and M. F. Sem.
FIG. 1 illustrates the population inversion mechanism, where E.sup.Z+ denotes the ground state of the Z.sup.th ionization stage of element E and E.sup.(Z+1)+ denotes the ground state of the (Z+1).sup.th ionization stage. During the heating phase of the plasma, step 1 in FIG. 1, a highly ionized plasma at high density is formed by an electrical discharge or a laser-produced plasma. At this time, the atoms of element E are excited, preferably within a gaseous medium, and some fraction are ionized into stage E.sup.(Z+1)+. After formation, the plasma is allowed to expand, illustrated as step 2 in FIG. 1. During this expansion phase, plasma electrons are cooled via collisions with the surrounding gases. As a consequence of this cooling the electron-ion recombination rate is significantly increased. The recombining electrons move downward in energy, via collisions with the free electron continuum, through the highly excited states of E.sup.Z+, step 3 in FIG. 1, until a significant gap in the energy levels of that charge state is reached. The reduced collisional decay rate across the gap creates a bottleneck, which bottleneck causes a population inversion to develop with respect to one or more lower levels. Laser action is achieved on transitions across the gap when a high decay rate exists for the lower level of such a transition.
The importance of the electron temperature on the electron-ion collisional recombination process is well known. The total three-body recombination rate for hydrogen-like ions (.tau..sub.3).sup.-1 is given by the equation found in Physics of Shock Waves and High Temperature Hydrodynamic Phenomena, Vol. I, Academic Press, New York, 1966, by Ya. B. Zeldovich and Yu. P. Raizer EQU (.tau..sub.3).sup.-1 =8.75.times.10.sup.-27 Z.sup.3 N.sub.e.sup.2 T.sub.e.sup.-4.5, (1)
where Z is the nuclear charge, N.sub.e is the electron density and T.sub.e is the electron temperature. This collisional recombination process is much more sensitive to temperature, T.sub.e.sup.-4.5, than is the radiative recombination process, T.sub.e.sup.-0.75. In fact, when a plasma is completely ionized, the plasma will not relax unless the plasma electrons are allowed to cool. An article entitled, "Collisional-Radiative Transfer Between Rydberg States of Helium and Electronic Recombination of He.sup.+ ", Phys. Rev. A, Vol. 15, 1977, pp. 1502-1512 by J. Boulmer, F. Devos, J. Stevefelt and J. F. Delpech discloses that the emission from a recombining plasma can be quenched by heating the electrons with an external source. In fact, an article entitled, "Ultra-High-Gain Laser-Produced Plasma Laser in Xenon Using Periodic Pumping", Appl. Phys. Lett. , Vol. 34, 1979, pp. 213-215, by W. T. Silfvast, L. H. Szeto and O. R. Wood discloses the fact that the plasma cooling rate has a dramatic effect on the output from a CO.sub.2 laser-produced plasma recombination laser in xenon.